Preheat oven to 375 degrees F. Heat a large heavy oven-proof skillet over medium-high heat. Add 2 Tablespoons olive oil. When hot, add onion and saute until just soft. Add garlic and saute for 1 more minute. Add potatoes, salt, pepper and berbere and saute and toss until the potaotes begin to brown. Remove from heat and add lentils and broth and stir to blend. Butterfly the chicken by cutting along both sides of the backbone with kitchen shears or a sharp knife. Turn the chicken over and flatten it with your hand to crack the breastbone. You can also cut along the breastbone to make two pieces. Coat the chicken with olive oil and sprinkle with salt and pepper. Place the chicken, skin side up, on top of the lentil mixture in the skillet and transfer to the oven, uncovered. Roast for 45 minutes then increase the temperature to 450 degrees and roast for another 30 minutes. Remove from oven and cover skillet loosely with foil and let stand for 10 minutes. Serve.
*Berbere is an aromatic Ethiopian spice that can be found in specialty food stores.